Communication channels on spacecraft or satellites may be dedicated for use between particular ground stations, in which case transmissions may take place continuously from one of the stations to the other. Bidirectional transmissions may take place if some means is provided for separating the signals, such as frequency-division multiple access. When multiple ground stations use the same spacecraft channel for communications among themselves, some arrangement must be provided for preventing simultaneous transmission of two or more signals through the same spacecraft channel, because such signals may interact or interfere in the channel, so that information is lost. In one prior-art approach, each transmission must be terminated before the next one is transmitted through the channel. This mode of use is known as time-division multiple access, or TDMA. In contrast, a random access scheme known as "Aloha" allows each ground station to transmit packets at random times in a mode which probabilistically may be in contention with other stations, with the result that, when there are many users of the channel, many of the transmitted packets interfere. Aloha provides a rescheduling protocol which enhances the likelihood that later transmissions will traverse the channel without interference. One such Aloha system is described in U.S. Pat. No. 4,745,599, issued May 17, 1988 in the name of Raychaudhuri. The Aloha type systems suffer from the disadvantage that utilization of the spacecraft channel is essentially wasted during the time in which it processes signals which interfere. It should be noted that a spacecraft channel may partially overlap the frequency or frequencies of adjacent channels in "frequency re-use" operation.
The utilization of a spacecraft channel may be improved by synchronizing the transmissions from the various ground stations in such a manner that each packet arrives at the spacecraft at the beginning of one of a plurality of sequential "slot" intervals. Each slot interval is equal in time duration to the packet interval, so that a stream of packets arriving at the spacecraft from different ground stations, with each packet arriving at the time of the beginning of a different slot interval, will traverse the spacecraft channel in a continuous sequence, without times in which the spacecraft channel is unutilized, and without interference by one packet with the signal of the two adjacent packets. This greatly improves the utilization of the channel, but is difficult to implement because each ground station is at a different distance from the spacecraft, and therefore signals from each ground station have different propagation times to the spacecraft. In addition, because the spacecraft moves relative to the ground stations, the propagation delays change with time. Some TDMA schemes provide a slot which is longer than the duration of a data packet, so that slight timing errors do not cause a mistimed data packet to interfere with the communication in an adjacent time slot. Such "guard times," however, reduce the throughput of the channel in proportion to their length, and it is therefore desirable to minimize the guard time. Known methods for reducing required guard intervals generally consume spacecraft power and bandwidth in proportion to the number of active ground stations in a network so that the cost in terms of data-carrying capacity becomes significant in large networks. Therefore, an improved method for synchronization control of a communications spacecraft channel is desired.